Plating metal negative electrodes under protective coatings

ABSTRACT

A method for forming lithium electrodes having protective layers involves plating lithium between a lithium ion conductive protective layer and a current collector of an “electrode precursor.” The electrode precursor is formed by depositing the protective layer on a very smooth surface of a current collector. The protective layer is a glass such as lithium phosphorus oxynitride and the current collector is a conductive sheet such as a copper sheet. During plating, lithium ions move through the protective layer and a lithium metal layer plates onto the surface of the current collector. The resulting structure is a protected lithium electrode. To facilitate uniform lithium plating, the electrode precursor may include a “wetting layer” which coats the current collector.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of U.S. patent applicationSer. No. 09/139,603 filed Aug. 25, 1998. This application is related toU.S. patent application Ser. No. 09/139,601 (Attorney Docket No.PLUSP015) filed on Aug. 25, 1998, titled “METHOD FOR FORMINGENCAPSULATED LITHIUM ELECTRODES HAVING GLASS PROTECTIVE LAYERS,” andnaming Steven J. Visco and Floris Y. Tsang as inventors. Thisapplication is also related to U.S. patent application Ser. No.09/086,665. Each of the above patent applications is incorporated hereinby reference for all purposes.

BACKGROUND OF THE INVENTION

[0002] This invention relates to negative electrodes for use inbatteries (e.g., lithium electrodes for use in lithium-sulfurbatteries). More particularly, this invention relates to methods offorming alkali metal electrodes having a thin glassy or amorphousprotective layer.

[0003] In theory, some alkali metal electrodes could provide very highenergy density batteries. The low equivalent weight of lithium rendersit particularly attractive as a battery electrode component. Lithiumprovides greater energy per volume than the traditional batterystandards, nickel and cadmium. Unfortunately, no rechargeable lithiummetal batteries have yet succeeded in the market place.

[0004] The failure of rechargeable lithium metal batteries is largelydue to cell cycling problems. On repeated charge and discharge cycles,lithium “dendrites” gradually grow out from the lithium metal electrode,through the electrolyte, and ultimately contact the positive electrode.This causes an internal short circuit in the battery, rendering thebattery unusable after a relatively few cycles. While cycling, lithiumelectrodes may also grow “mossy” deposits which can dislodge from thenegative electrode and thereby reduce the battery's capacity.

[0005] To address lithium's poor cycling behavior in liquid electrolytesystems, some researchers have proposed coating the electrolyte facingside of the lithium negative electrode with a “protective layer.” Suchprotective layer must conduct lithium ions, but at the same time preventcontact between the lithium electrode surface and the bulk electrolyte.Many techniques for applying protective layers have not succeeded.

[0006] Some contemplated lithium metal protective layers are formed insitu by reaction between lithium metal and compounds in the cell'selectrolyte which contact the lithium. Most of these in situ films aregrown by a controlled chemical reaction after the battery is assembled.Generally, such films have a porous morphology allowing some electrolyteto penetrate to the bare lithium metal surface. Thus, they fail toadequately protect the lithium electrode.

[0007] Various preformed lithium protective layers have beencontemplated. For example, U.S. Pat. No. 5,314,765 (issued to Bates onMay 24, 1994) describes an ex situ technique for fabricating a lithiumelectrode containing a thin layer of sputtered lithium phosphorusoxynitride (“LIPON”) or related material. LIPON is a glassy single ionconductor (conducts lithium ion) which has been studied as a potentialelectrolyte for solid state lithium microbatteries that are fabricatedon silicon and used to power integrated circuits (See U.S. Pat. Nos.5,597,660, 5,567,210, 5,338,625, and 5,512,147, all issued to Bates etal.).

[0008] In both the in situ and ex situ techniques for fabricating aprotected lithium electrode, one must start with a smooth clean sourceof lithium on which to deposit the protective layer. Unfortunately, mostcommercially available lithium has a surface roughness that is on thesame order as the thickness of the desired protective layer. In otherwords, the lithium surface has bumps and crevices as large as or nearlyas large as the thickness of the protective layer. As a result, mostcontemplated deposition processes cannot form an adherent gap-freeprotective layer on the lithium surface.

[0009] Thus, lithium battery technology still lacks an effectivemechanism for protecting lithium negative electrodes.

SUMMARY OF THE INVENTION

[0010] The present invention provides an improved method for formingactive metal electrodes having protective layers. Active metals includethose metals that can benefit from a protective layer when used aselectrodes. The method involves plating the active metal between aprotective layer and a current collector on an “electrode precursor.”The electrode precursor is formed by depositing the protective layer ona very smooth surface of a current collector. Because the surface onwhich the protective layer is deposited is very smooth, the protectivelayer has a higher quality than when deposited directly on thick lithiummetal. During plating, active metal ions move through the protectivelayer and an active metal layer plates onto the surface of the currentcollector. The resulting structure is a protected active metalelectrode. To facilitate uniform plating, the electrode precursor mayinclude a “wetting layer” which coats the current collector.

[0011] One aspect of the invention provides a method of fabricating analkali metal electrode, which method may be characterized by thefollowing sequence: (a) providing an alkali metal electrode precursor toan electrochemical cell, which electrode precursor includes a currentcollector and a glassy or amorphous protective layer forming asubstantially impervious layer which is a single ion conductorconductive to ions of an alkali metal; and (b) plating the alkali metalthrough the protective layer to form a layer of the alkali metal betweenthe current collector and the protective layer to form the alkali metalelectrode. Preferably, the alkali metal electrode precursor alsoincludes a wetting layer located between and adherent to the currentcollector and the protective layer. The wetting layer facilitates evendeposition of the alkali metal on the current collector. Note thatcurrent collectors are typically inert to the alkali metal and thereforedo not provide good plating surfaces. Often the alkali metal platesunevenly over the surface. In a preferred embodiment, the wetting layereither (i) intercalates alkali metal ions conducted by the single ionconductor or (ii) alloys with the alkali metal having ions conducted bythe single ion conductor.

[0012] The alkali metal may be plated in situ or ex situ. In the in situcase, a battery is assembled from the electrode precursor and otherbattery elements including an electrolyte and a positive electrode. Theelectrode precursor is then converted to an alkali metal electrode by aninitial charging operation in which lithium plates from the positiveelectrode. The battery may be either a primary or secondary battery.Prior to the plating step, such batteries do not contain free alkalimetal. This allows for safe transportation and long shelf life. Onlywhen a battery cell is ready for use is it charged for the first time toform the alkali metal electrode. Only then does it contain free alkalimetal.

[0013] In the ex situ case, the electrode is formed in an electrolyticcell that is separate from the battery in which it is ultimatelyassembled. Thereafter the electrode is removed from the electrochemicalcell and assembled into a battery.

[0014] The present invention also relates to alkali metal electrodeprecursors which may be characterized by the following features: (a) acurrent collector; (b) a glassy or amorphous protective layer forming asubstantially impervious layer which is a single ion conductorconductive to ions of an alkali metal; and (c) a wetting layer locatedbetween and adherent to the current collector and the protective layer.As mentioned in the method aspect of this invention, the wetting layereither (i) intercalates alkali metal ions conducted by the single ionconductor or (ii) alloys with the alkali metal having ions conducted bythe single ion conductor.

[0015] The current collector is typically a layer of metal such ascopper, nickel, stainless steel, or zinc. Alternatively, it may be ametallized plastic sheet or other metallized insulating sheet. If thewetting layer material alloys with the alkali metal, it may be silicon,magnesium, aluminum, lead, silver, or tin, for example. If the wettinglayer intercalates ions of the alkali metal, it may be carbon, titaniumsulfide, or iron sulfide, for example.

[0016] If the alkali metal is lithium, the protective layer should beconductive to lithium ions. Examples of suitable lithium ion conductingprotective layer materials include lithium silicates, lithium borates,lithium aluminates, lithium phosphates, lithium phosphorus oxynitrides,lithium silicosulfides, lithium borosulfides, lithium aluminosulfides,and lithium phosphosulfides. Specific examples of protective layermaterials include 6LiI—Li₃PO₄—P₂S₅, B₂O₃—LiCO₃—Li₃PO₄, LiI—Li₂O—SiO₂,and Li_(x)PO_(y)N_(z) (LIPON). Preferably, the protective layer has athickness of between about 50 angstroms and 5 micrometers (morepreferably between about 500 angstroms and 2000 angstroms). Preferably,the protective layer has a conductivity (to an alkali metal ion) ofbetween about 10⁻⁸ and about 10⁻² (ohm-cm)⁻¹.

[0017] As noted, the electrodes and electrode precursors of thisinvention may be assembled into alkali metal batteries. In a specificembodiment, the invention provides alkali metal batteries that may becharacterized by the following features: (a) a positive electrodecomprising a source of mobile alkali metal ions on charge; (b) aprecursor to an alkali metal negative electrode as described above; and(c) an electrolyte. Preferably, the alkali metal is at least one oflithium and sodium. The electrolyte may be liquid, polymer, or gel. In aparticularly preferred embodiment, the positive electrode includes atleast one of sulfides of the alkali metal, polysulfides of the alkalimetal.

[0018] Examples of suitable primary batteries include lithium manganesedioxide batteries, lithium (CF)_(x) batteries, lithium thionyl chloridebatteries, lithium sulfur dioxide batteries, lithium iron sulfidebatteries (Li/FeS₂), lithium polyaniline batteries, and lithium iodinebatteries. Examples of suitable secondary batteries includelithiumsulfur batteries, lithium cobalt oxide batteries, lithium nickeloxide batteries, lithium manganese oxide batteries, and lithium vanadiumoxide batteries. Other batteries employing active metals other thanlithium may be employed as well. These include the other alkali metalsand alkaline earth metals.

[0019] These and other features of the invention will be furtherdescribed and exemplified in the drawings and detailed descriptionbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is a schematic illustration of the steps associated with afirst preferred embodiment of the invention including forming anelectrode precursor and converting it to an electrode by plating lithiumon a current collector.

[0021]FIG. 2 is a schematic illustration of the steps associated with asecond preferred embodiment of the invention including forming anelectrode precursor and converting it to an electrode by plating lithiumon a wetting layer provided on a current collector.

[0022]FIG. 3 is a block diagram of a battery formed from an electrode ofthe present invention.

[0023]FIG. 4 is a schematic illustration of the oxidation states of asulfur catholyte during in situ lithium electrode formation andsubsequent cycling.

[0024]FIG. 5 is a graph of cell potential versus state of charge for asulfur catholyte of a lithium-sulfur cell.

[0025]FIG. 6 is a graph illustrating that after twenty charge/dischargecycles, nearly all of the lithium in an electrode prepared according tothis invention remained in the electrode.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] Using an Electrode Precursor

[0027] In the following description, the invention is presented in termsof certain specific compositions, configurations, and processes to helpexplain how it may be practiced. The invention is not limited to thesespecific embodiments. For example, while much of the followingdiscussion focuses on lithium systems, the invention pertains morebroadly to other active metal battery systems as well (e.g., batterieshaving negative electrodes of alkali metals and alkaline earth metals).

[0028]FIGS. 1 and 2 illustrate two preferred approaches to the presentinvention. Considering FIG. 1 first, a current collector 14 is provided.This should be a conductive material with at least a top surface that isvery smooth. On this smooth surface, a protective layer 18 is formed bya suitable process such as sputtering or chemical vapor deposition.Protective layer 18 should be a single ion conductor which conducts ionsof the active metal used in the electrode (e.g., lithium). Becauseprotective layer 18 is deposited on a very smooth surface, it too willbe smooth and continuous.

[0029] The resulting structure is referred to herein as an “electrodeprecursor” 17. It may be transported, stored, and otherwise handledwithout the precautions normally required for a lithium metal electrode.

[0030] Finally, lithium metal is electroplated onto current collector 14from a lithium ion source to produce a lithium electrode 10. Lithiumions move through protective layer 18 and contact current collector 14where they are reduced to form lithium metal. Thus, electrode 10includes a lithium metal layer 16 sandwiched between current collector14 and protective layer 18. Because the lithium layer is formed afterthe protective layer (rather than having the protective layer formed onthe lithium as in conventional processes), the protective layer is ofhigh quality. That is, the protective layer is generally gap-free andadherent when produced according to this invention.

[0031]FIG. 2 illustrates another preferred embodiment of the presentinvention. This approach may be appropriate when the current collectoris made from a material that does not allow lithium to plate evenly.Copper current collectors, for example, do not provide good surfaces forplating lithium. Lithium tends to plate on copper in discontinuouspatches. To address this problem, a “wetting layer” may be formed on thecurrent collector to reduce the surface energy at the interface of theplated lithium and the current collector.

[0032] As shown in FIG. 2, an electrode precursor 17′ is created fromcurrent collector 14, protective layer 18, and a wetting layer 15. Notethat wetting layer 15 is sandwiched between current collector 14 andprotective layer 18. Like electrode precursor 17, electrode precursor17′ may be handled and stored without the special precautions affordedalkali metal electrodes.

[0033] An electrode 10′ is formed by plating a lithium layer 16 ontowetting layer 15 and current collector 14. Thus, electrode 10′ comprisesa stack including a current collector as the bottom layer, a single ionconducting protective layer as the top layer, a wetting layer on thecurrent collector, and a lithium metal layer between the wetting layerand the protective layer. Wetting layer 15 may, but need not, integrateinto lithium layer 16 during plating. For example, if wetting layer 15is deposited as a layer of aluminum, it may form a lithium/aluminumalloy when lithium layer 16 is formed.

[0034] Note that in both electrodes 10 and 10′, current collector 14includes a first surface which is exposed to the ambient and a secondsurface which intimately contacts the lithium layer 16 (or possiblywetting layer 15). Lithium layer 16 includes a first surface which formsthe interface with current collector 14 (or possibly wetting layer 15)and a second surface which intimately contacts protective layer 18. Inturn, protective layer 18 includes a first surface which contacts thesecond surface of lithium layer 16. Finally, protective layer 18includes a second surface which is exposed to the ambient. Theinterfaces at the surfaces of lithium layer 16 should be sufficientlycontinuous or intimate that moisture, air, electrolyte, and other agentsfrom the ambient are prevented from contacting the lithium metal. Inaddition, the interface the lithium and the current collector shouldprovide a low resistance electronic contact.

[0035] Preferably, the current collectors employed with this inventionform a physically rigid layer of material that does not alloy withlithium. They should be electronically conductive and unreactive tomoisture, gases in the atmosphere (e.g., oxygen and carbon dioxide),electrolytes and other agents they are likely to encounter prior to,during, and after fabrication of a battery. Examples of materials usefulas current collectors for this invention include copper, nickel, manyforms of stainless steel, zinc, chromium, and compatible alloys thereof.The current collector should not alloy with, easily migrate into, orotherwise detrimentally affect the electrochemical properties of lithiumlayer 16. This also ensures that the current collector material does notredistribute during the charge and discharge cycles in which lithium isalternately plated and electrolytically consumed. In a preferredembodiment, the current collector may have a thickness of between about1 and 25 micrometers (more preferably between about 6 and 12micrometers).

[0036] In an alternative embodiment, the current collector is providedas a metallized plastic layer. In this case, the current collector maybe much thinner than a free-standing current collector. For example, themetal layer on plastic may be in the range of 500 angstroms to 1micrometer in thickness. Suitable plastic backing layers for use withthis type of current collector include polyethylene terephthalate (PET),polypropylene, polyethylene, polyvinylchloride (PVC), polyolefins,polyimides, etc. The metal layers put on such plastic substrates arepreferably inert to lithium (e.g., they do not alloy with lithium) andmay include at least those materials listed above (e.g., copper, nickel,stainless steel, and zinc). One advantage of this design is that itforms a relatively lightweight backing/current collector for theelectrode.

[0037] In an alternative embodiment, current collector 14 is coated witha non-electronically conductive outer layer such as a second protectivelayer. In this embodiment, a current collector or terminal must still beaffixed to the lithium electrode. This may take the form of a metal tabor other electronically conductive member that extends beyond theprotective layers.

[0038] The current collector may be prepared by a conventional techniquefor producing current collectors. For example, current collectors may beprovided as sheets of the commercially available metals or metallizedplastics. The surfaces of such current collectors may be prepared bystandard techniques such as electrode polishing, sanding, grinding,and/or cleaning. At this point, the surface of the current collectorshould be smoother than the thickness of the protective glass layersubsequently deposited onto it. For example, a current collector with asurface roughness on the order of micrometers might not be suitable fordeposition of a 1000 angstrom layer of glass.

[0039] Alternatively, the current collector metals may be formed by amore exotic technique such as evaporation of the metal onto a substrate,physical or chemical vapor deposition of the metal on a substrate, etc.Such processes may be performed as part of a continuous process forconstructing the electrode. Other sub-processes employed in thecontinuous process might include subsequent deposition of an aluminumlayer (one example of a wetting layer) and a lithium layer. Each step inthe continuous process would be performed under vacuum.

[0040] While the material comprising the current collector is preferablyinert to lithium, this makes it somewhat difficult to deposit a smoothcohesive layer of lithium on the current collector. For this reason, thepresent invention may employ a layer of “wetting” material on thecurrent collector to facilitate an even deposition of lithium in asubsequent step.

[0041] A goal in using the wetting layer of this invention is to preventthe lithium from preferentially plating at one or a few locations whereit grows so thick that it cracks the glass protective layer. Thus,during the initial plating cycle, the lithium should plate evenly overthe current collector surface to avoid cracking.

[0042] The wetting material should be chosen to lower the energy ofplating. Various materials may serve this function. Two general classesof suitable materials include (1) the materials that alloy with lithiumand (2) materials that intercalate lithium. Examples of materialsfalling into the first class include silicon, magnesium, aluminum, lead,silver, and tin. Materials falling into the second class include carbon,titanium sulfide (TiS₂), and iron sulfide (FeS₂).

[0043] Regardless of which wetting material is chosen, only a rathersmall amount of it should be employed. If too much of this material ispresent, it can effect the electrochemical properties of the electrode.Each of these materials will effect the redox potential of theelectrodes. In some embodiments, the wetting layer is between about 50and 1000 angstroms thick.

[0044] The wetting material should be formed with as smooth a surface aspossible. The r.m.s. thickness of the beginning layer should be nogreater than the anticipated thickness of the glass layer to besubsequently deposited. Suitably smooth layers may be deposited byvarious processes. Examples of suitable processes include physical vapordeposition (e.g. evaporation or sputtering) of aluminum or magnesiumwetting layers. Alternatively, chemical vapor deposition may be used todeposit carbon, silicon, titanium sulfide, and iron sulfide. So long asthe thickness of the wetting layers are kept relatively thin, (e.g.,within 50 to 1000 angstroms thick), it generally will not get too bumpy.

[0045] Preferably, the wetting layer remains in place during successivecycles of the electrode. In most cases, the wetting material will remainbehind the protective layer because the protective layer will not beconductive to ions of the wetting layer. For example, if the protectivelayer is a single ion conductor for lithium and the wetting layer isaluminum, aluminum ions will not pass through the protective layer.Thus, the proper choice of a protective layer and a wetting layer willensure that the wetting layer does not migrate throughout the cellemploying the electrode. In addition, the wetting layer may be “locked”in place within the matrix of the current collector. Stated another way,the current collector may be chemically modified with a wettingmaterial. In a preferred embodiment, this is accomplished by having agraded composition near the surface of the current collector in whichthe concentration of the wetting layer material increases toward thesurface.

[0046] Protective layer 18 serves to protect the lithium metal in theelectrode during cell cycling. It should protect the lithium metal fromattack from the electrolyte and reduce formation of dendrites and mossydeposits. In addition, layer 18 should be substantially impervious toagents from the ambient. Thus, it should be substantially free of pores,defects, and any pathways allowing air, moisture, electrolyte, and otheroutside agents to penetrate though it to metal layer 16. In this regard,the composition, thickness, and method of fabrication may all beimportant in imparting the necessary protective properties to layer 18.These features of the protective layer will be described in furtherdetail below.

[0047] Preferably, protective layer 18 is so impervious to ambientmoisture, carbon dioxide, oxygen, etc. that a lithium electrode can behandled under ambient conditions without the need for elaborate dry boxconditions as typically employed to process other lithium electrodes.Because the protective layer described herein provides such goodprotection for the lithium (or other reactive metal), it is contemplatedthat electrode 10 (or 10′) may have a quite long shelf life outside of abattery. Thus, the invention contemplates not only batteries containingnegative electrode 10, but unused negative electrodes themselves. Suchnegative electrodes may be provided in the form of sheets, rolls,stacks, etc. Ultimately, they are integrated with other batterycomponents to fabricate a battery. The enhanced stability of thebatteries of this invention will greatly simplify this fabricationprocedure.

[0048] The protective layer should be a glass or amorphous material thatconducts lithium ion but does not significantly conduct other ions. Inother words, it should be a single ion conductor. It should also bestable for the voltage window employed in the cell under consideration.Still further it should be chemically stable to the electrolyte, atleast within the voltage window of the cell. Finally, it should have ahigh ionic conductivity for the lithium ion.

[0049] The protective layer may be formed directly on top of the wettinglayer by any suitable process. It can be deposited on the wetting layerby techniques such as physical vapor deposition and chemical vapordeposition. In a preferred embodiment, it is deposited by plasmaenhanced chemical vapor deposition (PECVD). Examples of suitablephysical vapor deposition processes include sputtering and evaporation(e.g., electron-beam evaporation). A PECVD technique is described inU.S. patent application Ser. No. 09/086,665, filed on May 19, 1998, andtitled PROTECTIVE COATINGS FOR NEGATIVE ELECTRODES, which was previouslyincorporated herein by reference.

[0050] The lithium or other active material is provided to the electrodeelectrochemically by electroplating it on the current collector/wettingagent behind the protective layer. This may be accomplished either exsitu or in situ. In the ex situ case, the electroplating takes place ina system that is separate from the final battery or cell in which theelectrode is used. Thus, the lithium electrode is preformed beforeintroduction into the battery. In the in situ case, the compositeincluding the current collector, wetting agent, and protective layer isassembled into a battery containing a fully discharged positiveelectrode. The fully discharged positive electrode contains all thelithium or other metal necessary to cycle the cell. After the cell isassembled, it undergoes a charge cycle in which lithium (or other metal)is driven from the positive electrode and placed onto the negativeelectrode behind the protective layer.

[0051] In the ex situ case, the current collector/wettingagent/protective layer composite is provided to an electrolytic solutioncontaining an electrolyte and a source of lithium ions (e.g., a metalliclithium source). The source of lithium ions and the electrode precursorserve as electrodes and are connected by a current source. If a metalliccurrent collector (i.e., one that does not have an insulating backingsuch as PET) is used, the exposed face of the metallic current collectormust be masked in order to prevent the lithium or other metal fromdepositing on it. The goal is to ensure that all lithium is platedthrough the protective layer and onto the side of the current collectorhaving the wetting agent.

[0052] The electrolyte is preferably a high conductivity organicsolvent. It should be made as conductive as possible to increase theefficiency of the plating operation. The more conductive the material,the less energy required to plate the lithium or other metal onto thecomposite electrode. Examples of suitable electrolytes might includealkylene carbonates such as dimethyl carbonate, ethylene carbonate andpropylene carbonate, ethers such as monoglyme CH₃(OCH₂CH₂)OCH₃, diglymeCH₃(OCH₂CH₂)₂OCH₃, triglyme CH₃(OCH₂CH₂)₃OCH₃, tetraglymeCH₃(OCH₂CH₂)₃OCH₃, tetrahydrofuran, and polyethers such as polyethyleneoxide, dimethyl sulfoxide, sulfolane, tetraethyl sulfonamide, dimethylformamide, diethyl formamide, dimethyl acetamide, etc. Other suitablesolvents are known in the art. Usually the solvent will include aconductivity enhancing agent, such as lithiumtrifluoromethylsulfonimide.

[0053] During the plating operation, the composite electrode on whichthe lithium is to be plated is made negative and the source of lithiumelectrode is made positive. The current is controlled until a definednumber of Coulombs are passed. This defined number is set to correspondto the amount of lithium that is to be plated. The current determineshow fast the lithium is plated. Preferably, it is plated as fast aspossible without causing the protective layer to crack, lose adherence,or otherwise lose its protective function. When possible, it may bedesirable to perform the plating operation at a relatively hightemperature (e.g., between about 50 and 100 degrees Centigrade) in orderto increase electrolyte conductivity and thereby speed the platingprocess.

[0054] In the in situ case, the lithium necessary for forming thenegative electrode is obtained from the cathode or catholyte where itmay be safely held for long periods of time. In this approach, the cellis constructed essentially the same as it would be with a normal lithiumelectrode. However, there is no free lithium in the negative electrodeprior to the first charge cycle. The completed cell is in the dischargedstate. Because there is no free lithium metal present in the fullyassembled cell (before the initial charge), such cells may be safelystored for long periods of time and safely transported without reductionof shelf life.

[0055] Protective Layer Composition

[0056] Protective layer 18 is preferably composed of a glass oramorphous material that is conductive to alkali metal ions of the alkalimetal comprising layer 16. Preferably, protective layer 18 does notconduct anions such as S₈ ⁼ generated on discharge of a sulfur electrode(or other anions produced with other positive electrodes), or anionspresent in the electrolyte such as perchlorate ions from dissociation oflithium perchlorate.

[0057] In order to provide the needed ionic conductivity, the protectivelayer typically contains a mobile ion such as an alkali metal cation ofthe negative electrode metal. Many suitable single ion conductors areknown. Among the suitable glasses are those that may be characterized ascontaining a “modifier” portion and a “network former” portion. Themodifier is often an oxide of the alkali metal in layer 16 (i.e., themetal ion to which protective layer 18 is conductive). The networkformer is often a polymeric oxide or sulfide. One example is the lithiumsilicate glass 2 Li₂O.1 SiO₂ and another example is the sodiumborosilicate glass 2 Na₂O.1 SiO₂.2B₂O₃.

[0058] The modifier/network former glasses employed in this inventionmay have the general formula (M₂O)X(A_(n)D_(m)), where M is an alkalimetal, A is boron, aluminum, silicon, or phosphorous, D is oxygen orsulfur. The values of n and m are dependent upon the valence on A. X isa coefficient that varies depending upon the desired properties of theglass. Generally, the conductivity of the glass increases as the valueof X decreases. However, if the value of X becomes too small, separatephases of the modifier and network former arise. Generally, the glassshould remain of a single phase, so the value of X must be carefullychosen.

[0059] The highest concentration of M₂O should be that which yields thestoichiometry of the fully ionic salt of the network former. Forinstance SiO₂ is a polymeric covalent material; as Li₂O is added tosilica O—O bonds are broken yielding Si—O Li⁺. The limit of Li₂Oaddition is at the completely ionic stoichiometry, which for silicawould be Li₄SiO₄, or 2Li₂OSiO₂ (Li₂O.0.5SiO₂). Any addition of Li₂Obeyond this stoichiometry would necessarily lead to phase separation ofLi₂O and Li₄SiO₄. Phase separation of a glass composition typicallyhappens well before the fully ionic composition, but this is dependenton the thermal history of the glass and cannot be calculated fromstoichiometry. Therefore the ionic limit can be seen as an upper maximumbeyond which phase separation will happen regardless of thermal history.The same limitation can be calculated for all network formers, i.e.Li₃BO₃ or 3 Li₂O.B₂O₃, Li₃AlO₃ or 3 Li₂OAl₂O₃, etc. Obviously, theoptimum values of X will vary depending upon the modifier and networkformer employed.

[0060] Examples of the modifier include lithium oxide (Li₂O), lithiumsulfide (Li₂S), lithium selenide (Li₂Se), sodium oxide (Na₂O), sodiumsulfide (Na₂S), sodium selenide (Na₂Se), potassium oxide (K₂O),potassium sulfide (K₂S), potassium selenide (K₂Se), etc., andcombinations thereof. Examples of the network former include silicondioxide (SiO₂), silicon sulfide (SiS₂), silicon selenide (SiSe₂), boronoxide (B₂O₃), boron sulfide (B₂S₃), boron selenide (B₂Se₃), aluminumoxide (Al₂O₃), aluminum sulfide (Al₂S₃), aluminum selenide (Al₂Se₃),phosphorous pentoxide (P₂O₅), phosphorous pentasulfide (P₂S₅),phosphorous pentaselenide (P₂Se₅), phosphorous tetraoxide (PO₄),phosphorous tetrasulfide (PS₄), phosphorous tetraselenide (PSe₄), andrelated network formers.

[0061] “Doped” versions of the above two-part protective glasses mayalso be employed. Often the dopant is a simple halide of the ion towhich the glass is conductive. Examples include lithium iodide (LiI),lithium chloride (LiCl), lithium bromide (LiBr), sodium iodide (NaI),sodium chloride (NaCl), sodium bromide (NaBr), etc. Such doped glassesmay have general formula (M₂O)X(A_(n)D_(m)).Y(MH) where Y is acoefficient and MH is a metal halide.

[0062] The addition of metal halides to glasses is quite different thanthe addition of metal oxides or network modifiers to glasses. In thecase of network modifier addition, the covalent nature of the glass isreduced with increasing modifier addition and the glass becomes moreionic in nature. The addition of metal halides is understood more interms of the addition of a salt (MH) to a solvent (the modifier/formerglass). The solubility of a metal halide (MH) in a glass will alsodepend on the thermal history of the glass. In general it has been foundthat the ionic conductivity of a glass increases with increasing dopant(MH) concentration until the point of phase separation. However, veryhigh concentrations of MH dopant may render the glass hygroscopic andsusceptible to attack by residual water in battery electrolytes,therefore it might be desirable to use a graded interface where thehalide concentration decreases as a function of distance from thenegative electrode surface. One suitable halide doped glass isLi₂O.YLiCl.XB₂O₃.ZSiO₂.

[0063] Some other single ion conductor glasses may also be employed as aprotective layer used with this invention. One example is a lithiumphosphorus oxynitride glass referred to as LIPON which is described in“A Stable Thin-Film Lithium Electrolyte: Lithium Phosphorus Oxynitride,”J. Electrochem. Soc., 144, 524 (1997) and is incorporated herein byreference for all purposes. An example composition for LIPON isLi₂₉PO_(3.3)N_(0.5). Examples of other glass films that may work include6LiI—Li₃PO₄—P₂S₅ and B₂O₃—LiCO₃—Li₃PO₄.

[0064] Regarding thickness, protective layer 18 should be as thin aspossible while still effectively protecting the metal electrode. Thinnerlayers have various benefits. Among these are flexibility and low ionicresistance. If a layer becomes too thick, the electrode cannot bendeasily without cracking or otherwise damaging the protective layer.Also, the overall resistance of the protective layer is a function ofthickness. However, the protective layer should be sufficiently thick toprevent electrolyte or certain aggressive ions from contacting theunderlying alkali metal. The appropriate thickness will depend upon thedeposition process. If the deposition process produces a high qualityprotective layer, then a rather thin layer can be employed. A highquality protective layer will be smooth and continuous and free of poresor defects that could provide a pathway for lithium metal or deleteriousagents from the electrolyte.

[0065] For many protective layers, the optimal thickness will rangebetween about 50 angstroms and 5 micrometers. More preferably, thethickness will range between about 100 angstroms and 3,000 angstroms.Even more preferably, the thickness will range between about 500angstroms and 2,000 angstroms. For many high quality protective layers,an optimal thickness will be approximately 1000 angstroms.

[0066] In addition, the composition of the protective layer should havean inherently high ionic conductivity (e.g., between about 10⁻⁸ andabout 10⁻² (ohm-cm)⁻¹). Obviously, if a relatively good quality thinlayer can be deposited, a material with a relatively low conductivitymay be suitable. However, if relatively thicker layers are required toprovide adequate protection, it will be imperative that the compositionof the protective layer have a relatively high conductivity.

[0067] Battery Design

[0068] Batteries of this invention may be constructed according tovarious known processes for assembling cell components and cells.Generally, the invention finds application in any cell configuration.The exact structure will depend primarily upon the intended use of thebattery unit. Examples include thin film with porous separator, thinfilm polymeric laminate, jelly roll (i.e., spirally wound), prismatic,coin cell, etc.

[0069] Generally, batteries employing the negative electrodes of thisinvention will be fabricated with an electrolyte. It is possible,however, that the protective layer could serve as a solid stateelectrolyte in its own right. If a separate electrolyte is employed, itmay be in the liquid, solid (e.g., polymer), or gel state. It may befabricated together with the negative electrode as a unitary structure(e.g., as a laminate). Such unitary structures will most often employ asolid or gel phase electrolyte.

[0070] The negative electrode is spaced from the positive electrode, andboth electrodes may be in material contact with an electrolyteseparator. Current collectors contact both the positive and negativeelectrodes in a conventional manner and permit an electrical current tobe drawn by an external circuit. In a typical cell, all of thecomponents will be enclosed in an appropriate casing, plastic forexample, with only the current collectors extending beyond the casing.Thereby, reactive elements, such as sodium or lithium in the negativeelectrode, as well as other cell elements are protected.

[0071] Referring now to FIG. 3, a cell 310 in accordance with apreferred embodiment of the present invention is shown. Cell 310includes a negative current collector 312 which is formed of anelectronically conductive material. The current collector serves toconduct electrons between a cell terminal (not shown) and a negativeelectrode 314 (such as lithium) to which current collector 312 isaffixed. Negative electrode 314 is made from lithium or other similarlyreactive material, and includes a protective layer 308 formed oppositecurrent collector 312. It contacts current collector 312 via a wettinglayer 313. Either negative electrode 314 or protective layer 308contacts an electrolyte in an electrolyte region 316. As mentioned, theelectrolyte may be liquid, gel, or solid (e.g., polymer). To simplifythe discussion of FIG. 3, the electrolyte will be referred to as “liquidelectrolyte” or just “electrolyte.”

[0072] An optional separator in region 316 prevents electronic contactbetween the positive and negative electrodes. A positive electrode 318abuts the side of separator layer 316 opposite negative electrode 314.As electrolyte region 316 is an electronic insulator and an ionicconductor, positive electrode 318 is ionically coupled to butelectronically insulated from negative electrode 314. Finally, the sideof positive electrode 318 opposite electrolyte region 316 is affixed toa positive current collector 320. Current collector 320 provides anelectronic connection between a positive cell terminal (not shown) andpositive electrode 318.

[0073] Current collector 320, which provides the current connection tothe positive electrode, should resist degradation in the electrochemicalenvironment of the cell and should remain substantially unchanged duringdischarge and charge. In one embodiment, the current collectors aresheets of conductive material such as aluminum or stainless steel. Thepositive electrode may be attached to the current collector by directlyforming it on the current collector or by pressing a pre-formedelectrode onto the current collector. Positive electrode mixtures formeddirectly onto current collectors preferably have good adhesion. Positiveelectrode films can also be cast or pressed onto expanded metal sheets.Alternately, metal leads can be attached to the positive electrode bycrimp-sealing, metal spraying, sputtering or other techniques known tothose skilled in the art. Some positive electrode can be pressedtogether with the electrolyte separator sandwiched between theelectrodes. In order to provide good electrical conductivity between thepositive electrode and a metal container, an electronically conductivematrix of, for example, carbon or aluminum powders or fibers or metalmesh may be used.

[0074] A separator may occupy all or some part of electrolytecompartment 316. Preferably, it will be a highly porous/permeablematerial such as a felt, paper, or microporous plastic film. It shouldalso resist attack by the electrolyte and other cell components underthe potentials experienced within the cell. Examples of suitableseparators include glass, plastic, ceramic, and porous membranes thereofamong other separators known to those in the art. In one specificembodiment, the separator is Celgard 2300 or Celgard 2400 available fromHoechst Celanese of Dallas, Tex.

[0075] In an alternative embodiment, no separator is employed. Theprotective layer on the negative electrode prevents the positive andnegative electrodes from contacting one another and serves the functionof a separator. In such cases, the protective layer should be tough. Itmay be relatively thick and made from a material that resists crackingand abrasion.

[0076] In some embodiments of the invention, the cell may becharacterized as a “thin film” or “thin layer” cell. Such cells possessrelatively thin electrodes and electrolyte separators. Preferably, thepositive electrode is no thicker than about 300 μm, more preferably nothicker than about 1501 μm, and most preferably no thicker than about100 μm. The negative electrode preferably is no thicker than about 100μm and more preferably no thicker than about 100 μm. Finally, theelectrolyte separator (when in a fully assembled cell) is no thickerthan about 100 μm and more preferably no thicker than about 40 μm.

[0077] The present invention can be used with any of a number of batterysystems employing a highly reactive negative electrode such as lithiumor other alkali metal. For example, any positive electrode used withlithium metal or lithium ion batteries may be employed. These includelithium manganese oxide, lithium cobalt oxide, lithium nickel oxide,lithium vanadium oxide, etc. Mixed oxides of these compounds may also beemployed such as lithium cobalt nickel oxide. As will be explained inmore detail below, a preferred application of the electrodes of thisinvention is in lithium-sulfur batteries.

[0078] While the above examples are directed to rechargeable batteries,the invention may also find application in primary batteries. Examplesof such primary batteries include lithium-manganese oxide batteries,lithium-(CF)_(x) chloride batteries, lithium sulfur dioxide batteriesand lithium iodine batteries. These batteries would normally havelithium plated ex situ, and then have a long shelf life due to theprotective layer.

[0079] The protective layer allows one to use a reactive lithium metalelectrode in a manner that resembles the use of lithium ion batteries.Lithium ion batteries were developed because they had a longer cyclelife and better safety characteristics than metal lithium batteries. Therelatively short cycle life of metallic lithium batteries has been due,in part, to the formation of dendrites of lithium which grow from thelithium electrode across the electrolyte and to the positive electrodewhere they short circuit the cells. Not only do these short circuitsprematurely kill the cells, they pose a serious safety risk. Theprotective layer of this invention prevents formations of dendrites andthereby improves the cycle life and safety of metallic lithiumbatteries. Further, the batteries of this invention will perform betterthan lithium ion batteries because they do not require a carbonintercalation matrix to support lithium ions. Because the carbon matrixdoes not provide a source of electrochemical energy, it simplyrepresents dead weight that reduces a battery's energy density. Becausethe present invention does not employ a carbon intercalation matrix, ithas a higher energy density than a conventional lithium ion cell—whileproviding better cycle life and safety than metallic lithium batteriesstudied to date. In addition, the lithium metal batteries of thisinvention do not have a large irreversible capacity loss associated withthe “formation” of lithium ion batteries.

[0080] Lithium-sulfur Batteries

[0081] Sulfur positive electrodes and metal-sulfur batteries aredescribed in U.S. Pat. No. 5,686,201 issued to Chu on Nov. 11, 1997 andU.S. patent application Ser. No. 08/948,969 naming Chu et al. asinventors, filed on Oct. 10, 1997. Both of these documents areincorporated by reference for all purposes. The sulfur positiveelectrodes preferably include in their theoretically fully charged statesulfur and an electronically conductive material. At some state ofdischarge, the positive electrode will include one or more polysulfidesand possibly sulfides, which are polysulfides and sulfides of the metalor metals found in the negative electrode. In some embodiments, thefully charged electrode may also include some amount of such sulfidesand/or polysulfides.

[0082] The positive electrode is fabricated such that it permitselectrons to easily move between the sulfur and the electronicallyconductive material, and permits ions to move between the electrolyteand the sulfur. Thus, high sulfur utilization is realized, even aftermany cycles. If the lithium-sulfur battery employs a solid or gel stateelectrolyte, the positive electrode should include an electronicconductor (e.g., carbon) and an ionic conductor (e.g., polyethyleneoxide) in addition to the sulfur electroactive material. If the batteryemploys a liquid electrolyte, the positive electrode may require only anelectronic conductor in addition to the sulfur electroactive material.The electrolyte itself permeates the electrode and acts as the ionicconductor. In the case of a liquid electrolyte cell, the battery designmay assume two formats: (1) all active sulfur (elemental sulfur,polysulfides and sulfides of the positive electrode) is dissolved inelectrolyte solution (one phase positive electrode) and (2) the activesulfur is distributed between a solid phase (sometimes precipitated) anda liquid phase.

[0083] When the metal-sulfur battery cells of this invention include aliquid electrolyte, that electrolyte should keep many or all of sulfurdischarge products in solution and therefore available forelectrochemical reaction. Thus, they preferably solubilize lithiumsulfide and relatively low molecular weight polysulfides. In aparticularly preferred embodiment, the electrolyte solvent has repeatingethoxy units (CH₂CH₂O). This may be a glyme or related compound. Suchsolvents are believed to strongly coordinate lithium and therebyincrease the solubility of discharge products of lithium-sulfurbatteries. Suitable liquid electrolyte solvents are described in moredetail in U.S. patent application Ser. No. 08/948,969, previouslyincorporated by reference.

[0084] It should be understood that the electrolyte solvents of thisinvention may also include cosolvents. Examples of such additionalcosolvents include sulfolane, dimethyl sulfone, dialkyl carbonates,tetrahydrofuran (THF), dioxolane, propylene carbonate (PC), ethylenecarbonate (EC), dimethyl carbonate (DMC), butyrolactone,Nmethylpyrrolidinone, dimethoxyethane (DME or glyme),hexamethylphosphoramide, pyridine, N,N-diethylacetamide,N,N-diethylformamide, dimethylsulfoxide, tetramethylurea,N,N-dimethylacetamide, N,N-dimethylformamide, tributylphosphate,trimethylphosphate, N,N,N′,N′-tetraethylsulfamide, tetraethylenediamine,tetramethylpropylenediamine, pentamethyldiethylenetriamine, methanol,ethylene glycol, polyethylene glycol, nitromethane, trifluoroaceticacid, trifluoromethanesulfonic acid, sulfur dioxide, boron trifluoride,and combinations of such liquids.

[0085] The protective layers employed in this invention may allow theuse of electrolyte solvents that work well with sulfides andpolysulfides but may attack lithium. Examples of solvents in thiscategory include amine solvents such as diethyl amine, ethylene diamine,tributyl amine, amides such as dimethyl acetamide and hexamethylphosphoramide (HMPA), etc.

[0086] Exemplary but optional electrolyte salts for the battery cellsincorporating the electrolyte solvents of this invention include, forexample, lithium trifluoromethanesulfonimide (LiN(CF₃SO₂)₂), lithiumtriflate (LiCF₃SO₃), lithium perchlorate (LiClO₄), LiPF₆, LiBF₄, andLiAsF₆, as well as corresponding salts depending on the choice of metalfor the negative electrode, for example, the corresponding sodium salts.As indicated above, the electrolyte salt is optional for the batterycells of this invention, in that upon discharge of the battery, themetal sulfides or polysulfides formed can act as electrolyte salts, forexample, M_(x/z)S wherein x=0 to 2 and z is the valence of the metal.

[0087] As mentioned, the battery cells of this invention may include asolid-state electrolyte. An exemplary solid-state electrolyte separatoris a ceramic or glass electrolyte separator which contains essentiallyno liquid. Specific examples of solidstate ceramic electrolyteseparators include beta alumina-type materials such as sodium betaalumina, Nasicon™ or Lisicon™ glass or ceramic. Polymeric electrolytes,porous membranes, or combinations thereof are exemplary of a type ofelectrolyte separator to which an aprotic organic plasticizer liquid canbe added according to this invention for the formation of a solid-stateelectrolyte separator generally containing less than 20% liquid.Suitable polymeric electrolytes include polyethers, polyimines,polythioethers, polyphosphazenes, polymer blends, and the like andmixtures and copolymers thereof in which an appropriate electrolyte salthas optionally been added. Preferred polyethers are polyalkylene oxides,more preferably, polyethylene oxide.

[0088] In the gel-state, the electrolyte separator generally contains atleast 20% (weight percentage) of an organic liquid (see the above listedliquid electrolytes for examples), with the liquid being immobilized bythe inclusion of a gelling agent. Many gelling agents such aspolyacrylonitrile, polyvinylidene difluoride (PVDF), or polyethyleneoxide (PEO), can be used.

[0089] It should be understood that some systems employing liquidelectrolytes are commonly referred to as having “polymer” separatormembranes. Such systems are considered liquid electrolyte systems withinthe context of this invention. The membrane separators employed in thesesystems actually serve to hold liquid electrolyte in small pores bycapillary action. Essentially, a porous or microporous network providesa region for entraining liquid electrolyte. Such separators aredescribed in U.S. Pat. No. 3,351,495 assigned to W. R. Grace & Co. andU.S. Pat. Nos. 5,460,904, 5,540,741, and 5,607,485 all assigned toBellcore, for example. Each of these patents is incorporated herein byreference for all purposes.

[0090] The fully charged state of some cells of this invention need notrequire that the positive electrode be entirely converted to elementalsulfur. It may be possible in some cases to have the positive electrodebe a highly oxidized form of lithium polysulfide, for example, as inLi₂S_(x) where x is five or greater. The fully charged positiveelectrode may also include a mixture of such polysulfides together withelemental sulfur and possibly even some sulfide. It should be understoodthat during charge, the positive electrode would generally not be ofuniform composition. That is, there will be some amount of sulfide,sulfur, and an assortment of polysulfides with various values of x.Also, while the electrochemically active material includes somesubstantial fraction of “sulfur,” this does not mean that the positiveelectrode must rely exclusively upon sulfur for its electrochemicalenergy.

[0091] The electronic conductor in the positive electrode preferablyforms an interconnected matrix so that there is always a clear currentpath from the positive current collector to any position in theelectronic conductor. This provides high availability of electroactivesites and maintained accessibility to charge carriers over repeatedcycling. Often such electronic conductors will be fibrous materials suchas a felt or paper. Examples of suitable materials include a carbonpaper from Lydall Technical Papers Corporation of Rochester, N.H. and agraphite felt available from Electrosynthesis Company of Lancaster, N.Y.

[0092] The sulfur is preferably uniformly dispersed in a compositematrix containing an electronically conductive material. Preferredweight ratios of sulfur to electronic conductor in the sulfur-basedpositive electrodes of this invention in a fully charged state are atmost about 50:1, more preferably at most about 10:1, and most preferablyat most about 5:1. The sulfur considered in these ratios includes bothprecipitated or solid phase sulfur as well as sulfur dissolved in theelectrolyte. Preferably, the per weight ratio of electronic conductor tobinder is at least about 1:1 and more preferably at least about 2:1.

[0093] The composite sulfur-based positive electrode may furtheroptionally include performance enhancing additives such as binders,electrocatalysts (e.g., phthalocyanines, metallocenes, brilliant yellow(Reg. No. 3051-11-4 from Aldrich Catalog Handbook of Fine Chemicals;Aldrich Chemical Company, Inc., 1001 West Saint Paul Avenue, Milwaukee,Wis.) among other electrocatalysts), surfactants, dispersants (forexample, to improve the homogeneity of the electrode's ingredients), andprotective layer forming additives to protect a lithium negativeelectrode (e.g., organosulfur compounds, phosphates, iodides, iodine,metal sulfides, nitrides, and fluorides). Preferred binders (1) do notswell in the liquid electrolyte and (2) allow partial but not completewetting of the sulfur by the liquid electrolyte. Examples of suitablebinders include Kynar available from Elf Atochem of Philadelphia, Pa.,polytetrafluoroethylene dispersions, and polyethylene oxide (of about900k molecular weight for example). Other additives includeelectroactive organodisulfide compounds employing a disulfide bond inthe compound's backbone. Electrochemical energy is generated byreversibly breaking the disulfide bonds in the compound's backbone.During charge, the disulfide bonds are reformed. Examples oforganodisulfide compounds suitable for use with this invention arepresented in U.S. Pat. Nos. 4,833,048 and 4,917,974 issued to DeJongheet al. and U.S. Pat. No. 5,162,175 issued to Visco et al.

[0094] The battery cells of this invention may be rechargeable“secondary” cells. Unlike primary cells which discharge only once, thesecondary cells of this invention cycle between discharge and charge atleast two times. Typically, secondary cells of this invention will cycleat least 50 times, with each cycle having a sulfur utilization (measuredas a fraction of 1675 mAh/g sulfur output during the discharge phase ofthe cycle) of at least about 10%. More preferably, at least 50 cycleswill have a minimum sulfur utilization of at least about 20% (mostpreferably at least about 30%). Alternatively, the secondary cells ofthis invention will cycle at least two times, with each cycle attainingat least 50% utilization of sulfur in the positive electrode.

[0095] The use of an in situ process for forming a lithium electrode ina lithium-sulfur cell in accordance with this invention provides asimple mechanism for controlling the oxidation state of the positiveelectrode during normal cell cycling. For example, the designer maydesign the cell so that very little if any highly reduced species suchas lithium sulfide are produced during cycling. This may be accomplishedby using a relatively highly oxidized species (e.g., Li₂S₂) as thesource of lithium (positive electrode) for in situ lithium negativeelectrode formation. Since all the lithium in the cell originally camefrom Li₂S₂, the positive electrode of a fully discharged cell will havespecies whose average oxidation state corresponds to Li₂S₂. Thisprevents formation of Li₂S in any significant quantity. Note that highlyreduced species such as Li₂S are relatively insoluble in comparison tomore highly oxidized species and therefore may be undesirable withliquid electrolyte cells. That is, many cell designs require that mostor all sulfur species remain in solution. If sulfur reduces in aninsoluble form (e.g., Li₂S or a related species), it may lose electricalcontact with the electrode/current collector and thereby becomeunavailable for electrochemical reaction. This reduces the cell'scapacity and energy density.

[0096]FIG. 4 schematically illustrates the general concept ofcontrolling the oxidation state of the positive electrode. Initially, acell 401 is assembled. It includes a negative electrode precursor 402, aseparator 409, and a cathode/catholyte 411. Catholyte 411 is a solutionof Li₂S₄ which permeates through separator 409 to contact protectivelayer 407. It also contacts a positive electrode 415 which may be acarbon mesh or mat in contact with an aluminum current collector.Electrode precursor 402 includes a current collector 403 (e.g., a coppersheet), a wetting layer 405 (e.g., aluminum), and a lithium conductingglass protective layer 407.

[0097] When cell 401 is ready for its initial use, it is charged to formthe lithium electrode. This results by applying a negative potential tocurrent collector 403 and a positive potential to current collector 415.Positively charged lithium ions leave the Li₂S₄, pass through glass ionconductor 407, and reduce to lithium metal on current collector 403 andthere alloy with aluminum from layer 405. As shown in FIG. 4, a lithiumelectrode 402′ forms. It includes a lithium layer 413 located betweenglass protective layer 407 and current collector 403. It contactscurrent collector 403 through an Al/Li alloy 405′.

[0098] The charging that produces lithium electrode 402′ also oxidizesthe catholyte species to a higher average oxidation state than Li₂S₄.For example, it may produce a charged catholyte 411′ which containshighly oxidized polysulfide species such as Li₂S₈ as well as possiblyelemental sulfur.

[0099] Fully charged lithium-sulfur 401 may be discharged to produceuseful electrochemical energy. During discharge, negative electrode 402′is gradually oxidized and the lithium metal in layer 413 converts tolithium ions which move through protective layer 407 and into catholyte411′. The highly oxidized polysulfides and elemental sulfur are reducedby reaction with the lithium ions liberated by negative electrode 402′.As a result, the catholyte species decrease in average oxidation state.

[0100] As shown in FIG. 4, the normal discharged state of cell 401includes discharged negative electrode 402″ and catholyte 411. Note thatelectrode 402″ no longer includes lithium layer 413 as it has beenconsumed. As a result electrode 402″ includes current collector 403,Li/Al wetting layer 405″, and protective layer 407. Catholyte 411includes reduced polysulfide species such as Li₂S₄. Note that in theoriginal cell, Li₂S₄ was chosen for the catholyte so that the sulfurcompounds always have a relatively high oxidation state (greater thanLi₂S) even during full discharge. Thus, all sulfur species tend toremain in solution, as they never reach an oxidation state approachingLi₂S.

[0101] Subsequent charge/discharge cycles convert the negative electrodebetween charged state 402′ in which a layer of lithium 413 forms anddischarged state 402″ in which some or all of layer 413 is consumed.That same cycling converts the catholyte between charged state 411′ inwhich oxidized species such as elemental sulfur and Li₂S₈ form anddischarged state 411 in which reduced species such as Li₂S₄ which form.However, strongly reduced (less soluble) species such as Li₂S do notform.

[0102]FIG. 5 is a graph 501 of cell potential versus state of charge fora sulfur catholyte of a typical lithium-sulfur cell. The cell voltage(abscissa) is a function of the state of charge (ordinate) of the sulfurcathode/catholyte. The slope of the graph reflects the fact thatdifferent sulfur containing species have different redox potentialsversus lithium species. A lithium-sulfur cell having 100 percent sulfuras the cathode/catholyte will have a cell voltage of approximately 2.5volts. As that cell discharges, the catholyte state of charge (andcomposition) changes so that polysulfides are formed and the cellpotential decreases. The potential curve has a shoulder at acathode/catholyte average composition of about Li₂S₈ as illustrated inthe graph. The curve remains relatively flat during further reduction incharge state until the average composition reduces below about Li₂S₂, atwhich point the potential drops rapidly until the fully reduced state(Li₂S) is reached.

[0103] A lithium sulfur cell design may limit range of the Li—Spotential curve over which the cell operates between charge anddischarge. The size of the range depends upon the relative amounts oflithium and sulfur in the system. When the relative amount of lithium islow the potential range of the cell is narrow. When the relative amountof lithium is high the potential range of the cell is large.

[0104] Bars 503 and 505 represent two different lithium-sulfur cells.Cell 503 has a relatively small range implying a relatively low ratio ofLi:S, while cell 505 has a wider range implying a higher ratio of Li:S.In situ cells in which the initial source of lithium is a lithium richhighly reduced sulfur species (e.g., Li₂S), the potential range isgreater.

[0105] In addition, the relative position of the potential range on theoverall Li—S potential curve depends upon the composition of thestarting lithium source. More highly oxidized species (e.g., a mixtureincluding Li₂S₄ and Li₂S₆) in the lithium source provide cells whichoperate further to the left on the potential curve of FIG. 4.

EXAMPLES

[0106] The following results have been observed:

[0107] 1. In an aprotic solvent in the presence of lithium polysulfide,as in the catholyte described, shiny lithium can be electroplatedthrough the LIPON layer onto the copper surface (example 1). Undersimilar conditions, electroplating onto uncovered copper foil did notyield any lithium coating.

[0108] 2. This lithium-plated LIPON/copper structure is a useable anode.Example 2 demonstrates its cyclability in the presence of the catholyte.

[0109] 3. This plated lithium is also essentially totally anode-active.Example 3 demonstrates the availability of lithium after twenty cyclesis at least 98% of the original anode-active lithium.

[0110] An experimental system was prepared as follows. A LIPON/coppercomposite was made by sputtering LIPON on a copper film, similar to aprocess described by Bates et. al. in Solid State Ionics, 53-56 (1992),647-654 which is incorporated herein by reference for all purposes. ALIPON target was made via simple fusion. A LIPON target pre-form wasmade by heating 11.5 gm. of lithium phosphate powder purchased from AlfaAesar to 1250 deg.C (80 deg. C/min heating rate) in a 95/5 Pt/Aucrucible (diameter 34 mm. at base, wall slightly tapered), holding atthat temperature for 15 minutes, followed by cooling at 80 deg C/minuntil the temperature dropped below 300 deg.C. This pre-form was sizedto a desirable shape with a “medium” Drywall Screen by 3M. The weight ofthe finished target was about 9.5 gm.

[0111] A 1.3″ Minimak sputtering head (manufactured by US Inc.) poweredby a RF10 power supply (by RF Plasma Products) was used to sputter theLIPON onto Cu foil (0.01 mm thick, by Schlank). The particularconditions for this run were 20 milli-torr nitrogen, at a nitrogen flowrate of 20 sccm, RF power of 75 watts forward, 0 watt reflected, targetto substrate distance of 8 cm, and duration of 58 minutes. Under suchconditions of sputtering, apparently LIPON was reactively formed on thesubstrate surface.

[0112] A cells were constructed with the LIPON/Cu anode precursor, apolypropylene separator (0.58 mm thick, by Hollensworth Vose) which alsoserves as the catholyte reservoir, and a C/Al cathode current collector.The Cu piece was about 2 cm.×2 cm, while the other components are about1 cm×1 cm. The copper side was placed on the anode lead plate, while astainless steel plate (ca 1 cm×1 cm), under a light spring tension,served as the cathode contact. The cells, unless otherwise noted, werefilled with a catholyte, a lithium sulfide solution in tetraglymecontaining 3 moles sulfur and 0.75 moles lithium/liter, with 0.5moles/liter lithium trifluoromethanesulfonimide as supportingelectrolyte. The cells were designed to seal easily and be disassembledeasily. Electrical maneuvering and measurements were performed on aMaccor cell cycler.

EXAMPLES

[0113] 1. Cell #D042 was constructed as described above. It was chargedat 100 microamperes for one hour. It was then disassembled. A uniform,shiny lithium film on the copper surface could easily be seen throughthe transparent LIPON glass layer. A parallel experiment was conductedin which the only variance was that the copper foil was not LIPONcoated. This did not yield any lithium coat, either by voltageindications during charging or by physical observation upon disassembly.

[0114] 2. Cell #D049 was constructed as described above. It was chargedat 100 microamperes for 2 hours. It was then cycled at 100 microamperesfor 15 minutes discharge followed by 15 minutes charge, for 100 cycles.Except for a minimal rise and fall of apparent cell internal resistance,much of which is attributable to the change of ambient temperature, thecell behaved remarkably constantly throughout the cycling.

[0115] 3. Cell #D111 was constructed as described above, except thesputtering process was conducted for 85 minutes at 78 watts forwardpower. It was initially electrically determined to be void of metalliclithium on the copper anode surface. The cell was then charged at 10microamperes for 10 min, then at 100 microamperes for 2 hours, thencycled at 100 microamperes, 15 min discharge/15 min charge, for 20cycles. The cell was then put on a strip mode, when it was discharged at100 microamperes until the closed circuit potential reaches 2.0 V, atwhich time the current was decreased to 20 microamperes, to a closedcircuit potential of 2.0 V. This stripping step integrated to 188.2microampere-hr., indicating a minimum of 188.2 microampere -hr ofanode-active Li remained at the end of cycling. In other words, after atotal of 201.6 microampere-hr of initially plated lithium was cycled 20times at 25 microampere-hr per cycle, a minimum of 188.2 microampere-hrremained available. To put it another way, a total of 701.6 (201.6+20cycles×25/cycle) microamperes/hr of Li was electroplated onto the copperanode, and at least 688.2 (188.2+20 cycles×25/cycle) microampere-hr ofdischarge was logged. See FIG. 6. The term “minimum” must be stressed,since obviously, from the definition of the endpoint of the strip stepused, there still is unused lithium available at the end of the stripstep.

[0116] Other Embodiments

[0117] The foregoing describes the instant invention and its presentlypreferred embodiments. Numerous modifications and variations in thepractice of this invention are expected to occur to those skilled in theart. For example, the invention may provide overcharge protection asdescribed in U.S. patent application Ser. No. 08/686,609, filed Jul. 26,1996, and entitled RECHARGEABLE POSITIVE ELECTRODES and U.S. patentapplication Ser. No. 08/782,245, filed Mar. 19, 1997, and entitledOVERCHARGE PROTECTION SYSTEMS FOR RECHARGEABLE BATTERIES. Suchmodifications and variations are encompassed within the followingclaims.

[0118] All references cited herein are incorporated by reference for allpurposes.

What is claimed is:
 1. A method of fabricating an electrode, the methodcomprising: (a) providing an electrode precursor in an electrochemicalcell, the electrode precursor including a current collector, a materialthat intercalates ions of an alkali metal, and a glassy or amorphousprotective layer forming a substantially impervious layer which is asingle ion conductor conductive to ions of the alkali metal; and (b)electrolytically transporting ions of the alkali metal through theprotective layer to intercalate into the carbon to form the electrode.2. The method of claim 1 , wherein the electrochemical cell is adischarged battery and wherein electrolytically transporting the alkalimetal ions to form the electrode is an initial charging operation. 3.The method of claim 2 , wherein the battery is a primary battery.
 4. Themethod of claim 1 , further comprising (c) removing the electrode fromthe electrochemical cell.
 5. The method of claim 4 , further comprising(d) assembling a battery including the electrode.
 6. The method of claim1 , wherein the material that intercalates ions of the alkali metal isselected from the group consisting of carbon, titanium sulfide, and ironsulfide.
 7. The method of claim 6 , wherein the material thatintercalates ions of the alkali metal is carbon.
 8. An electrodeprecursor comprising: a current collector; a glassy or amorphousprotective layer forming a substantially impervious layer which is asingle ion conductor conductive to ions of an alkali metal; and amaterial that intercalates ions of the alkali metal located between andadherent to the current collector and the protective layer; a materialthat intercalates ions of the alkali metal located between and adherentto the current collector and the protective layer; wherein the electrodeprecursor does not comprise an alkali metal.
 9. The alkali metalelectrode precursor of claim 8 , wherein the current collector is alayer of metal.
 10. The alkali metal electrode precursor of claim 8 ,wherein the metal in the layer of metal is selected from the groupconsisting of copper, nickel, stainless steel, and zinc.
 11. The alkalimetal electrode precursor of claim 8 , wherein the current collector isa metallized plastic sheet.
 12. The alkali metal electrode precursor ofclaim 8 , wherein the material that intercalates ions of the alkalimetal is selected from the group consisting of carbon, titanium sulfide,and iron sulfide.
 13. The alkali metal electrode precursor of claim 8 ,wherein the material that intercalates ions of the alkali metal iscarbon.
 14. The alkali metal electrode precursor of claim 8 , whereinprotective layer is conductive to lithium ions.
 15. The alkali metalelectrode precursor of claim 8 , wherein protective layer includes atleast one of a lithium silicate, a lithium borate, a lithium aluminate,lithium oxide, a lithium phosphate, a lithium phosphorus oxynitride, alithium silicosulfide, a lithium borosulfide, a lithium aluminosulfide,and a lithium phosphosulfide.
 16. The alkali metal electrode precursorof claim 8 , wherein the protective layer has a thickness of betweenabout 50 angstroms and 5 micrometers.
 17. The alkali metal electrodeprecursor of claim 16 , wherein the protective layer has a thickness ofbetween about 500 angstroms and 2000 angstroms.
 18. The alkali metalelectrode precursor of claim 8 , wherein the protective layer has anionic conductivity of between about 10⁻⁸ and about 10⁻² (ohm-cm)⁻¹. 19.A battery or a battery precursor comprising: a) a positive electrodecomprising a source of mobile alkali metal ions on charge; b) aprecursor to an alkali metal negative electrode including a currentcollector, a glassy or amorphous protective layer forming asubstantially impervious layer which is a single ion conductorconductive to ions of an alkali metal, and a layer of material thatintercalates ions of the alkali metal located between and adherent tothe current collector and the protective layer; and c) an electrolyte;wherein the precursor does not include an alkali metal.
 20. The batteryor battery precursor of claim 19 , wherein the alkali metal comprises atleast one of lithium and sodium.
 21. The battery or battery precursor ofclaim 19 , wherein the protective layer includes at least one of alithium silicate, a lithium borate, a lithium aluminate, a lithiumphosphate, a lithium phosphor nitride, a lithium silicosulfide, alithium borosulfide, a lithium aluminosulfide, and a lithiumphosphosulfide.
 22. The battery or battery precursor of claim 19 ,wherein the protective layer has a thickness of between about 50angstroms and 3000 angstroms.
 23. The battery or battery precursor ofclaim 19 , wherein the electrolyte is a liquid electrolyte.
 24. Thebattery or battery precursor of claim 19 , wherein the electrolyte is apolymer or gel electrolyte.
 25. The battery or battery precursor ofclaim 19 , wherein the positive electrode includes an electrochemicallyactive metal oxide.
 26. A method of fabricating a battery, the methodcomprising: (a) providing an electrode precursor in an electrochemicalcell, the electrode precursor comprising (i) a current collector, (ii) aglassy or amorphous protective layer forming a substantially imperviouslayer which is a single ion conductor conductive to ions of an alkalimetal, and (iii) sandwiched between the current collector and theprotective layer, a material that intercalates ions of the alkali metal;(b) providing a positive electrode; and (c) transporting alkali metalions through the protective layer to intercalate alkali metal ions intothe intercalation material and form the battery.
 27. The method of claim26 , wherein the electrochemical cell is a discharged battery andwherein transporting the alkali metal ions to form the electrode is aninitial charging operation.
 28. The method of claim 27 , wherein thedischarged battery is a secondary battery.
 29. The method of claim 26 ,wherein the positive electrode is a metal oxide electrode.
 30. Themethod of claim 29 , wherein the positive electrode is selected from thegroup consisting of lithium manganese oxide, lithium cobalt oxide,lithium nickel oxide, lithium vanadium oxide, and mixed oxides of any ofthese compounds.